Electroless plating



Oct. 25, 1966 R. c. COLONEL ELECTROLESS PLATING Filed April 12, 1963 RELAY I x 500 I500 2500 I000 ZOOO BUBBLE RATE IN 00/ Min./ Gal.

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M .L m M m0 M? M5 P 3 4/ O .I r W E m T G 2 m mm P mufi N Ml L U M U C c A United States Patent 3,281,266 ELECTRO ESS PLATING Richard C. Colonel, Fridley, Minn., assignor to Honeywell Inc., a corporation of Delaware 3,281,266 Patented Oct. 25, 1966 While the following discussion will be limited to electroless nickel, it will be appreciated by those skilled in the art that the invention will find application in other electroless plating baths such as iron, cobalt, chromium and Filed Apr. 12, 1963, Ser. No. 272,696 5 t e like.

6 Claims. (Cl. 117-130) The following prior art electroless nickel plating solution will be used as a starting point in setting forth the The present invention is directed to an improved elecadvantages of the present invention over the prior art. troless plating method and is more particularly directed Other electroless nickel plating baths, as will be dcto an improved electroless plating wherein the plating 10 scribed subsequently, show similar results. rate is substantially faster than has been heretofore possible. Example I.Base bath Since the discovery of a practical electroless nickel Nickel sulfate plating system by Abner Brenner and Grace Riddell of heXahYdl'ate 41 grams/liter the Bureau of Standards, investigators have sought to Sodium hypophos-phite utilize electroless nickel plating for innumerable jobs dihydrate 13.2 grams/liter (0.106 mol/l.). where the unique characteristics of electroless plating may Sodium citrate 36.4 grams/liter (0.123 mol/l.). be used to advantage. However, while electroless plating Wetting agent possesses numerous advantages over conventional electro- (Sorbit A.C.) 0.8 grams/ liter. plating, it has heretofore had two major disadvantages; Ammonium acetate 15.5 grams/liter (0.2 mol/l.). the relatively slow plating rate (generally about 0.5 with pH 6.0-6.2. up to 1.0 mil/hour by special procedures) andthe short isorbit AC is Wetting agent supplied by Geigy Chemical platlng bath life before rejuvenation of the bath is necl a y- Use of a wet g age vaids in Obtaining a tightly essay illdylslelllfilnr-ltdgotgg ng of metal, although use of a wetting agent is Therefore, it is an object of the present invention to provide a method of electroless plating wherein markedly It will be obvious to those skilled in the art that many higher plating rates are obtained than known heretofore. variations of the above bath can be made as to selection It is a further object of the present invention to proof th ina tiv i n of the compounds, For example, Vide method of Plating which Permits nickel ion may also be supplied as nickel chloride or tended, operation Platmg bath Y reiuvenation other nickel salts. Similarly, other buffers and carboxylic i wlthout Prohlbltlve decrease In Pl'atmg rate with acid salts may be utilized. As these are matters Well me. i

other advantages and objects w be pp from llilroewn ln the art, they wlll not be further elaborated upon a study of the following specification and drawings i above bath was heated to F n F and an wherein l 1' t th t b ltd Th FIGURE 1 is an illustration of a plating bath in aca l Sampe f e e cordance with the present invention; and aluminum had previous been cleaned and pretreated FIGURE 2 is a plot of accumulated plating versus time to the followmg i i Whlle for various electroless plating baths of the prior art and a Preferred one for bta1 mng a substrate which the present invention; 40 Wlll give good adherence of the nlckel to the aluminum,

FIGURE 3 is a plot of plating rate per hour for 1, 2, 3 and 4 consecutive hours from the same plating bath without rejuvenation as a function of rate of bubbling of air through the solution.

other procedures, known in the plating art, may also be used. For purposes of this discussion, all samples were aluminum, although identical results will be obtained on any catalytic surface.

TABLE I.TEST SAMPLE PREPARATION Process Operation Solution Make-up Composition Time Temp.

Vapor degrease.-. Non-etch soak clean... Oakite 61A 105:1 min 160;i:5 Rinse Flowing tap water 10-12% B/W NaOH. 30:1;10 sec 145;|;5 Flowing tap water. 85;l;fi% B/V HNO 105:5 sec Room Flowing tap water ZnSO 720 g./l.. HF 30 mL/l 80 sec Room Flowing tap water. -75% B/V HNO; Until gassing stops. Room Flowing tap water 15-25% B/V H 30 1-2 min- 150:1:10 Flowing tap water Activation 90-10 B/V HNOa-HF 103:5 sec Room Rinse Flowing tap water Zinc immersion ZnSOl 720 g./1., HF 30 ml/.l. sec Room Rinse Flowing tap water Activati0n MnSO 40 g./l., H01 ml./l 1% to 2 min Room inse Flowing tap water. Electroless nickel plate See examples The area-volume relationship of the samples to be plated and the solution was equal to 32 square inches per gallon of solution (approximately 5 cm. 100 cc.). Preparation of specimens to be plated was identical in all cases.

Curve A of FIGURE 2 shows the thickness of accumulated plating on the test panel as a function of time. As can be seen, slightly less than 0.5 mil was deposited in the first hour. During subsequent plating in the same bath, only small quantities of metal were added to the first hours deposit.

Example II The bath used for plating was identical to that of Example I with the exception that 1500 cc./min./gallon of air was bubbled through the solution during the plating period. Referring to FIGURE 1, generally designates a plating bath system in accordance with the present invention. Numeral 11 designates a plating tank, which may be of glass, as shown, or of any suitable material commonly used in electroles-s plating. Numeral 12 designates the test panels immersed in plating solution 13. Control of temperature in the plating operation is important as the rate of plating is very sensitive to temperature. It is desirable to operate very close to boiling. To accomplish this control, a heater 15, temperature sensor 16, and relay 17 may be employed. A ring member 19 having a multiplicity of openings 18 is connected to a source of air or oxygen 20 as will be hereinafter described. Also shown is a source of distilled water 21 to replenish the water lost during plating by evaporation. This loss will be quite large due to the passage of air or oxygen through the solution. Panels were plated as in Example 1 except that air was passed through the solution at a rate of 1500 cc./min./ gal. of solution. Accumulated plating as a function of time is shown in FIGURE 2 as curve B. As can be seen, plating rate was somewhat slower than in the absence of bubbling of air through the solution.

Example III A third plating bath was prepared as in Example II with the exception that the sodium hypoph-osphite concentration was raised to 82.2 grams/liter (0.66 Incl/1.). NBHgPOg concentration in the past has been limited to concentration of less than about 30 grams/liter due to the fact that the plating bath becomes increasingly unstable as the H PO concentration is increased. Ultimately, a spontaneous decomposition of the bath takes place with creation of finely divided nuclei of nickel metal (black nickel) appearing throughout the bulk of the plating bath. This spontaneous decomposition is in contrast to the catalytic decomposition occurring normally on the surface of the object to be plated. I

In a copending application of Jerome S. Sallo, Serial No. 245,234, filed December 17, 1962, assigned to the same assignee as the present invention there is disclosed a method of stabilizing elect-roless plating bath system against such spontaneous decomposition by means of bubbling a small quantity of oxygen through the bath during plating. The use of bubbled oxygen in the plating bath as taught in the Sallo application permits increased H PO concentration without the spontaneous decomposition found in the use of high H PO concentration by investigators previous to Sallo. Care must be exercised in adding the H2PO2 to prevent decomposition from occurring during the formulation of the plating bath. I have found it desirable to have air or oxygen passing through the solution during mixing both as an aid to solution by the stirring action and as a stabilizing means to prevent the spontaneous decomposition. Of course, once the solution is prepared, and particularly when the solution is at elevated temperatures, the passage of air or 0 through the bath continuously is mandatory or the bath will rapidly decompose.

The plating bath, operating conditions, and sample preparation were the same in Example III with the exception of H2PO2 concentration (present as being 39 grams/liter and the air was bubbled through the solution at 3,000 cc./min./gal. of solution. Results are shown as curve D. As can be seen, the rate is quite rapid for the first two hours, but diminishes sharply thereafter.

Example V The plating bath, operation conditions, and sample preparation were the same as Example III with the exception that 0.5 mg./l. of thiourea was added to the solution. Plating rate is shown as curve E of FIGURE 2.

Example VI In all of the preceding examples the initial pH of the solution was from 6.0 to 6.2. However, even with the bufiering actiori of the salts of weak acids in the solution, it was found that the solution changed pHquite rapidly to about 5.2. ;The pH at this point remained relatively constant due to the buffering action. In this present example, the plating bath, operating conditions, and sample preparation were the same as previously described in Example V with the exception that a continuous adjustment of pH was made by the addition of a solution of NaOI-I to maintain the pH at 6.0 to 6.2. Plating rate is shown as curve F on FIGURE 2. As can beseen, the rate is markedly higher than in the instances when pH ,is not as rigorously controlled. Great care is necessary in adjusting pH at elevated temperatures due to the tendency of the bath to decompose when local pH is strongly altered by the addition of sodium hydroxide. The preferredmethod of adjusting pH is to addthe caustic solution at lower temperatures than the bath operating temperature.

Example VII In Example V thiourea was added to the plating bath of Example III with the improved results shown in curve E of FIGURE 2. In the present example the plating bath of Example III was utilized with the exception that air was bubbled through the solution at a rate of 2,000 cc./min./gal. and thioaoetamide was added in place of the thiourea as in Example V. The results obtained are shown in FIGURE 2 as curve G. No attempt was made to control the pH as in Example VI. As can be seen, the plating rate was markedly higher than in the preceding examples-at least during the first hour. Other thio type compounds are similar in their action.

Example VIII In all of the preceding examples air has been bubbled through the solution. Air is the cheapest and most readily available source of oxygen. However, one may also use pure oxygen in an analogous manner to air with very favorable results. For an illustration of the use of pure oxygen, a bath in accordance with Example V was prepared with the exception that area to volume was 12 in. /gal. and that 200 cc./min./ gal. of solution of pure 0 was bubbled therethrou-gh. Plating results are shown in FIGURE 2 as curve H. The favorable results are evident. Higher bubbling rates are permissible, but

the 0 at higher rates apparently tends to oxidize the hypophosphite ion and thus lowers plating rate below that shown in curve H-especially after the first hour.

Example IX As an illustration of the effect of using a gas containing less oxygen than air, a bath in accordance with Example V was used with the exception that a blend of 1,500 cc./min./ gal. of N and 500 cc./-min./gal. of air was 'bubbled through the plating bath. Results are shown in FIGURE 2 as curve I. Plating rate was very good during the first hour, but there was evidence of spontane ous decomposition of the bath during the second hour. At the end of the second hour the bath had spontaneously decomposed to the extent that plating was no longer possible. The cause would appear to be that the amount of 0 available was not adequate to maintain bath stability over a long plating cycle.

The results of the above samples clearly show the advantages to be gained from bubbling of air or oxygen through an electroless plating bath which has increased hypophosphite ion concentration over previously known baths. I have further found that the results obtained are dependent on the rate at which air or oxygen is bubbled through the bath as well as upon the hypophosphite ion concentration, the pH, and the presence of sulfur containing additives such as thiourea. The plating rate obtained the first hour of baths life is not significantly affected by the rate at which air or 0 is bubbled through as long as the rate of bubbling is sufiicient to prevent spon taneous decomposition and is greater than about 200 cc./min./gal. However, the plating rate is significantly affected after the first hour by the rate of bubbling of air through the solution. The following examples and the graph of FIGURE 3 will show that certain rates of bubbling of air through the solution are preferable over other rates when a plating bath is to be used for several consecutive hours without rejuvenation.

Example X In the following examples a plating bath of the following composition was used:

NiSO 6H O 41 grams/liter (0.15 mol./l.). NaH PO -2H O 83.3 grams/liter (0.67 mol./l). Sodium citrate 36.4 grams/liter (0.12 mol./l.). Ammonium acetate 15.5 grams/liter (0.2 moi/1.). pH 6.0-6.2.

Thiourea 0/5 mg./l.

Sorbit A.C 0.8 grams/l.

Aluminum panels were prepared as in Example I-IX. Area to volume relationship was 6.5 square inches per gallon of solution. Temperature was maintained at 200 Eil F., but pH was not rigorously controlled as in Example VI. Air was bubbled through several baths containing test panels at rates of 500, 1,000, 1,500, 2,000, 2,500 and 3,000 cc./min./gal. Thickness of the metal deposited was determined at one hour intervals and the thickness of the metal deposited during one hour periods is plotted in FIGURE 3 as a function of bubble rate. Although thickness values were obtained for bu'bble rates of 1,500 cc./min./ gallon or less up through the third hour it should be realized that some spontaneous decomposition was occurring in baths having these lower bubbling rates which would render the deposits obtained on the samples commercially unsuitable due toroughness. As indicated in FIGURE 3, the plating rate during the fourth hour was not obtainable for samples plated in baths when bubbling rate was less than 2,000 cc./min./ gallon due to the fact that spontaneous decomposition had progressed to the point where valid samples could no longer be obtained.

It is also evident that as the bubbling rate increases over 2,500 cc./ min./ gallon that plating rate begins to de- Example XI Thus far, the invention has been described principally with regard to the use of air as the gas to be used in bub bling. Cost considerations make the use of air particularly attractive, but oxygen can also be used advantageously as the present example will illustrate. The rather rapid bubbling found desirable with air produces rapid evaporation in the plating bath which can be troublesome to the commercial user of the present invention. By use of oxygen the necessary bubbling rate for high speed, long life plating baths can be considerably reduced.

A plating bath and test specimens were prepared as in Example X with the exception that oxygen was bubbled through the solution during plating at a rate of 200 cc./min./-gallon of solution. Plating thickness obtained was as follows:

At the end of: Thickness, mils First hour 2.38 Second hour 1.40 Third hour 1.60 Fourth hour 1.20

Total 6.58

Test results have shown that changing of various parameters of the plating bath such as nickel ion concentration, buffering agents, choice of carboxylic acid salts and the like will vary the plating rate obtainable using the present invention. Likewise, the concentration of hypophosphite ion concentration may be increased to 1.2 mol/liter or possibly higher. However, I have found that in each instance the plating rate and stability of these baths is improved over similar baths known heretofore.

I claim:

1. The method of chemically nickel plating a catalytic material which comprises preparing an aqueous bath consisting essentially of nickel ions, a thio compound, and hypophosphite ions, said hypophosphite ion concentration being in excess of about 0.3 molar and said thio compound selected from the group consisting of thiourea and thioacetamide, heating Said bath to near boiling while bubbling a gas from the group consisting of air and oxygen through said bath at a rate in excess of 200 cc./min./ gallon, immersing said material in said bath, and allowing said material to remain in said bath until the desired thickness of metal is deposited thereon.

2. The method of claim 1 wherein the concentration of hypophosphite ion is about 0.6 molar.

3. The method of chemically nickel plating a catalytic material which comprises preparing an aqueous bath consisting essentially of nickel ions, a thio compound, and hypophosphite ions, said hypophosphite ions concentration being in excess of about 0.3 molar, and said thio compound selected from the group consisting of thiourea and thioacetamide, heating said bath to near boiling while bubbling oxygen t-herethrough at a rate in excess of 200 cc./min./gallon, immersing said material in said bath, and allowing said material to remain in Said bath until the desired thickness of metal is deposited thereon.

4. The method of chemically nickle plating a catalytic material which comprises preparing an aqueous bath consisting essentially of nickel ions, a thio compound, and hypophosphite ions, said hypophosphite ions concentrations being in excess of about 0.3 molar, and said thio compound being selected from the group consisting of thiourea and thioacetamide, maintaining the pH of said bath in the range of 5.2 to 6.2, heating said bath to near boiling while bubbling air therethrough at a rate of from about 500 to about 3,000 cc./min./ gallon, immersing said material in said bath until the desired thickness of metal is deposited thereon.

7' 5. The method of chemically nickle plating a catalytic material which comprises preparing an aqueous bath of the following composition:

Ni++ 0.15 moL/liter.

H P0 0.3 m 0.7 mol./liter. 5

'Citrate 0.1'2 moL/liter.

Ammonium acetate 0.2 moL/liter. 2:721:81;

-pH s 6.0-6.2. 2,819,188

rThiourea 0.5 mg./1iter. 2,929,742 10 2,938,805

heating said'bath to near boiling While bubbling air therethrough at a rate of about 500 up to about 3,000 cc./rnin./gallon, immersing said material in said bath, and allowing said material to remain in" said bath until the de'sired thickness of metal is deposited'thereon.

UNITED 6, The method in accordance with claim 5 wherein said bubbling rate is about 2,500 cc./minu'te/ gallon.

References Cited by the Examiner STATES PATENTS Jendrzynski et a1. 117130 Metheny et al 117-130 De Minjer et a1. 117-130 Agens 117- l30 ALFRED L. LEAVITT, Primary Examiner.

RALPH S. KENDALL, Examiner. 

1. THE METHOD OF CHEMICALLY NICKEL PLATING A CATALYTIC MATERIAL WHICH COMPRISES PREPARING AN AQUEOUS BATH CONSISTING ESSENTIALLY OF NICKEL IONS, A THIO COMPOUND, AND HYPOPHOSPHITE IONS, SAID HYPOPHOSPHITE ION CONCENTRATION BEING IN EXCESS OF ABOUT 0.3 MOLAR AND SAID THIO COMPOUND SELECTED FROM THE GROUP CONSISTING OF THIOUREA AND THIOACETAMIDE, HEATING SAID BATH TO NEAR BOILING WHILE BUBBLING A GAS FORM THE GROUP CONSISTING OF AIR AND OXYGEN THROUGH SAID BATH AT A RATE IN EXCESS OF 200 CC./MIN./ GALLON, IMMERSING SAID MATERIAL IN SAID BATH, AND ALLOWING SAID MATERIAL TO REMAIN IN SAID BATH UNTIL THE DESIRED THICKNESS OF METAL IS DEPOSITED THEREON. 